Method and apparatus for continuously dispersing fine particle-sized solids in a liquid

ABSTRACT

A method and apparatus for continuously dispersing fine particle-sized solids in liquids by a continuous process, in which a constant stream of liquid is fed into a mixing apparatus and fine-particle-sized solids are metered into the mixing apparatus by means of a metering screw as a function of the pressure prevailing in the mixing apparatus. The solids conveyed by the metering screw are supplied from an intermediate vacuum tank with a controlled filler level.

BACKGROUND OF THE INVENTION

The present invention relates to a method for continuously dispersingfine-particle-sized solids in a liquid, and, in particular, a liquidcomponent of a two-component reactive plastic, such as for example apolycondensation or polyaddition system, and more particularly in thepolyol component of a polyurethane system.

In two-component plastic, the two plastic components which are reactivewith one another are usually mixed in liquid form in a mixhead inspecific quantitative ratios and then reacted, optionally after havingbeen introduced into a mold. Catalysts or heat are optionally used toaccelerate the reactions. In the case of polyurethane plastics, such asfor example polyurethane foams, one of the components consists of anisocyanate or an isocyanate prepolymer and the other component of apolyol, the two components being metered into the mixhead in a specificratio.

Inorganic, e.g. mineral fillers, or organic fillers are frequently addedto such two-component plastics for the purpose of modifying theirproperty profile. U.S. Pat. No. 5,152,943 (corresponding to GermanOffenlegungschrift 3,841,671) describes mixing the filler with one ofthe reactive components of the two-component system in a preliminarymixer and feeding the filler-containing component into the mixhead. Thefiller is fed into the preliminary mixer by means of a screw in whichthe filler is compacted, the metering of the filler being effected bycontrolling the speed of rotation of the screw as a function of thepressure in the preliminary mixer.

European Patent 431,388 describes preventing any undesirableintroduction of air into the preliminary mixer in the process accordingto the '943 patent by designing the jacket of the compacting screw atthe outlet end upstream of the preliminary mixer in the form of a porousmicrofilter consisting of a porous sintered metal or a plastic andsurrounding it with an annular suction channel for removing the air fromthe compacted filler by suction. The disadvantage of this process isthat when fine particle-sized fillers are used or fillers with a highcontent of very fine particles, the microfilter becomes blocked withinrelatively short periods of time and must be replaced.

The object of the present invention was therefore to prevent theabove-mentioned disadvantages. More particularly, the object was toprevent the undesirable air present in the intermediate spaces betweenthe solid particles from entering the preliminary mixer, whilemaintaining the metering accuracy of the metering screw for the fineparticle-sized solids (fillers). This object is achieved by the presentinvention.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a device according to the present invention.

DESCRIPTION OF THE INVENTION

The present invention is directed to a method for continuouslydispersing fine particle-sized solids in a liquid using a mixingapparatus, in which a specific stream of liquid is continuously fed intothe mixing apparatus and the fine particle-sized solids are fedcontinuously into the mixing apparatus by means of a screw in which theyare compacted. The metering of the solids is controlled by controllingthe conveying speed of the screw as a function of the pressureprevailing in the mixing apparatus and the solids are deaerated beforebeing introduced into the mixing apparatus. An intermediate vacuum tankis arranged upstream of the screw which supplies the solids to themixing apparatus, and the filler level in the intermediate tank is keptessentially constant. The height of the fine-particle-sized solids inthe intermediate tank is preferably at least twice to three times, andat most 7 to 10 times the diameter of the screw which supplies thefiller to the mixing apparatus.

According to the invention a filler level probe is provided in theintermediate vacuum tank, by means of which the amount of fineparticle-sized solids fed into the intermediate tank is controlled.Preferably the intermediate tank is fed from a storage vessel by meansof a feed screw or a rotary feeder. The feed screw or the rotary feederpreferably feeds the solids into the intermediate tank at approximatelythe same level as the filler level of the tank, in order to prevent thefine particle-sized filler from trickling or forming dust in theintermediate tank. Air is removed by suction from the top of theintermediate tank by means of a vacuum pump, so that reduced pressureprevails in the intermediate tank. The reduced pressure which prevailsin the intermediate tank and is controlled by the vacuum pump is suchthat the quantity of air remaining between the solid particles istolerable and can for example be completely dissolved in the liquid inthe mixing apparatus. The pressure prevailing in the intermediate tankcan be from 0.01 to 0.8 bars (absolute), and preferably from 0.1 to 0.3bars.

The intermediate tank preferably has such dimensions that the resultingresidence time of the fine particle-sized solids in the intermediatetank is from 2 to 10 minutes, and preferably from 3 to 5 minutes.

The method according to the invention is particularly suitable fordispersing fine particle-sized solids in liquids which only wet thesolids with some difficulty or in high viscosity liquids withviscosities of, for example, from 300 to 3,000 mPa·s, since it isparticularly difficult to remove any remaining undesirable gas contentafter the solid and the liquid have been mixed.

Preferably the binder component of a two-component reactive plastic isused as the liquid, and in particular the polyol component of apolyurethane reactive system. The organic or inorganic fillerscustomarily used in two-component systems can be used as the solids,such as, for example, ground glass fibers, hollow glass beads, finelydivided silicon dioxide, ground plastic waste materials and melamineresins. The process according to the invention is particularly suitablefor fillers with a substantial content of particles with particle sizesbelow 50 μ and in particular below 10 μ. Thus the process is extremelysuitable for dispersing fillers resins with average particle sizes offrom 30 to 80μ in the polyol component of a polyurethane reactivesystem.

The continuous dispersing method according to the invention isparticularly suitable for the continuous production of filler-containingpolyurethane plastics by the continuous process, in which the dispersionobtained in the preliminary mixer is fed directly to a mix head formixing the filler-containing polyol component with the isocyanatecomponent.

The present invention is also directed to an apparatus for continuouslydispersing fine particle-sized solids in a liquid by the continuousprocess, in which the fine particle-sized solids are deaerated beforebeing dispersed in the liquid. The apparatus broadly comprises a mixingapparatus, devices for feeding a constant stream of liquid to the mixingapparatus, an intermediate tank for the fine particle-sized solids,devices for maintaining reduced pressure in the intermediate tank, ascrew for conveying the fine-particle-sized solids from the intermediatetank to the mixing apparatus, devices for measuring the pressure in themixing apparatus, devices for controlling the conveying power of thescrew as a function of the pressure in the mixing apparatus, devices forfeeding fine particle-sized solids into the intermediate tank, as wellas devices for controlling the introduction of the solids into theintermediate tank in accordance with a specific required filler level inthe intermediate tank.

Reference will now be made to the drawing. A constant stream of liquid,e.g. the polyol component of a polyurethane reactive system, is fed intothe mixing apparatus 1 by means of metering pump 2 via line 3. At thesame time, a deaerated fine particle-sized filler is fed into the mixingapparatus 1 via screw 4. The metering of the filler is effected bykeeping constant the pressure measured in the mixing apparatus 1 by thepressure-measuring device P, by controlling the driving power of themotor M of the screw 4 via control device 5. The screw 4 conveys thesolids issuing from the intermediate tank 6. The intermediate tank 6 isfed with solids from the storage vessel 11 by means of screw 10, thedriving power of the motor M of the screw 10 being controlled in a sucha manner by the control device 9, as a function of the signal from thefiller level probe 7, that the filler level in the intermediate tank 6is kept approximately constant. The gas space above the filler level ofthe intermediate tank 6 is continuously deaerated by means of vacuumpump 8. In addition, the contents 12 of the intermediate tank 6 can beloosened by means of loosening devices 13. By means of pump 14 thepolyol/solids dispersion is fed directly to a mixhead--which is notillustrated--in which it is mixed with the isocyanate component.

Although the invention has been described in detail in the foregoing forthe purpose of illustration, it is to be understood that such detail issolely for that purpose and that variations can be made therein by thoseskilled in the art without departing from the spirit and scope of theinvention except as it may be limited by the claims.

What is claimed is:
 1. A method for continuously dispersing fineparticle-sized solids in a liquid using a mixing apparatus comprisingfeeding a constant stream of liquid into the mixing apparatus, feedingthe fine particle-sized solids into the mixing apparatus via a screw inwhich said fine particle-sized solids are compacted, controlling themetering of the solids by controlling the conveying speed of the screwas a function of the pressure prevailing in the mixing apparatus anddeaerating the solids in an intermediate vacuum tank before they areintroduced into the mixing apparatus, wherein said intermediate vacuumtank is arranged upstream of the screw which supplies the solids to themixing apparatus, and wherein the filler level of the fineparticle-sized solids in the intermediate tank is kept essentiallyconstant.
 2. A method of claim 1, wherein the filler level of the fineparticle-sized solids in the intermediate tank is at least twice tothree times the diameter of the screw.
 3. An apparatus for continuouslydispersing fine particle-sized solids in a liquid by a continuousprocess, in which the fine particle-sized solids are deaerated beforebeing dispersed in the liquid, said apparatus comprising a mixingapparatus, a device for feeding a constant stream of liquid into themixing apparatus, an intermediate tank for the fine particle-sizedsolids, a device for maintaining a reduced pressure in the intermediatetank, a screw for conveying the fine particle-sized solids from theintermediate tank to the mixing apparatus, a device for measuring thepressure in the mixing apparatus, a device for controlling the conveyingpower of the screw as a function of the pressure measured by saidpressure measuring device and wherein said controlling device isconnected to the conveying power for the screw, a device for conveyingthe fine particle-sized solids to the intermediate tank, and a devicefor controlling the introduction of the solids into the intermediatetank in accordance with a specific required solids level of theintermediate tank.
 4. The apparatus of claim 3, wherein the device forconveying the fine particle-sized solids to the intermediate tank isscrew conveyor.